How to set vertex radius when using skin modifier?
@lemon Was kind enough to help me find this answer:

def make_increase_width(self):
    size = self.properties.skin_size
    obj = bpy.context.active_object
    for iteration in self._iterations:
        size += self.properties.width_increase
    for iteration in self._iterations:
        for vert_index in self._iterations[iteration]:
            obj.data.skin_vertices[''].data[vert_index].radius = (size, size)
        size -= self.properties.width_increase


This is outside of edit mode, very fast and exactly what I needed.